The present invention relates to an infrared detector device of semiconductor material and the manufacturing process thereof.
As known, silicon is currently the main material for producing integrated electronic components, and a wide variety of electrical functions may be implemented using silicon-based devices.
A new optical communications technology is emerging wherein the elementary information is carried by optical signals. Wavelengths in optical communication technology are typically between 1.3 and 1.55 xcexcm. It is therefore desirable to bring optical and electronic functions together in a single silicon device, combining electronic and optical technologies.
To this end significant progress has been made in fabricating silicon optical devices operating at wavelengths in the near infrared range: in particular, low-loss optical waveguides have been developed; the producibility of light-emitting diodes of planar structure based on doping with erbium ions has been proved; and optical silicon switches based on an electro-optical effect have been fabricated. There are, however, no infrared silicon-based detectors operating at wavelengths greater than 1.1 xcexcm, the wavelength associated with the silicon band-gap. Silicon does not absorb light at wavelengths in excess of 1.1 xcexcm. This property may be advantageously exploited to produce transparent optical waveguides of low loss at wavelengths greater than 1.1 xcexcm, but it makes silicon unsuitable for detecting light in the infrared range.
Two approaches have been proposed for modifying the properties of silicon and allowing the production of an infrared detector: the deposition of germanium (Ge) or silicon-germanium (SixG1xe2x88x92x) layers on silicon, and the deposition of thin metal layers on silicon to form Schottky diodes and thus produce detectors operating by internal photoemission. Neither solution is satisfactory, however. Germanium or silicon-germanium layers are subject to mechanical stress and faults. Furthermore, the quantum efficiency of such devices at room temperature is low. Devices based on metal/semiconductor barriers also have low quantum efficiency at room temperature and they have high noise because of thermoionic emission.
It has also been demonstrated that ions of rare earths, incorporated into silicon in the trivalent state, exhibit well-defined electron transitions due to an incomplete 4f shell. For example, erbium incorporated in a trivalent state has a first excited state at 0.8 eV (corresponding to 1.54 xcexcm) with respect to the ground state. This transition energy depends on the specific rare earth ions (equal, for example, to approximately 1.2 eV for ytterbium Yb, 1.16 eV for holmium Ho and 1.37 eV for neodymium Nd). These transitions may be excited both optically and electrically by a charge carrier mediated process.
Hereinafter, reference is made to optical excitation that takes place when a photon with an energy resonant with the transition energy of rare earth ions produces the excitation of the ion from its ground state to its first excited state. This process is shown diagrammatically in FIGS. 1a-1c in the specific case of erbium. In FIG. 1a, a photon with an energy of 1.54 xcexcm and incident on an erbium-doped region is absorbed by and excites an erbium ion. The excited erbium ion may then become de-excited, transferring its energy to the semiconductor electron system. In the example shown, during de-excitation the erbium ion gives up its energy to an electron at the top of the valency band (energy Ev), bringing it to a defect level ET in the silicon band gap, also due to the energy contribution provided by the phonons supplying the extra energy required to reach level ET (FIG. 1b). This passage is thus more efficient at high temperature. It may then happen that the electron at the defect level absorbs thermal energy such as to cause it to pass from defect level ET to conduction band EC (FIG. 1c). Overall, in the process shown, absorption of a photon at 1.54 xcexcm leads to generation of a free hole-electron pair. This hole-electron pair may then be separated and gathered by the electric field present in the region accommodating the rare earth ion, thus giving rise to an electric current that may be detected and is directly proportional to the intensity of infrared light.
The above-described process for converting infrared light into electrical current has been demonstrated in silicon solar cells doped with erbium for which photocurrents have been obtained at wavelengths of approx. 1.54 xcexcm. The conversion efficiency in such cells is, however, very low, on the order of 1E-6, which is insufficient for implementation in commercial devices.
The invention provides an infrared detector exploiting the conversion principle above described and having greater efficiency than present efficiency.